One type of such image forming devices is a device which employs an ink jet recording system. In the ink jet recording system, a nozzle filled with ink from an ink tank is provided with a heater which is heated in response to a heating pulse signal to thereby produce an air bubble, the pressure of which acts to eject an ink drop from the nozzle. In an image forming device employing the ink jet recording system, a plurality of nozzles are disposed in a line to form a recording head which is scanned to record an image.
As shown in FIG. 9, a recording head 103 (hereinafter referred to as simply a head) mounted on a carriage is scanned in a main scanning direction (X) to perform a printing on a paper sheet 15 column by column (17), so that a multitude of such columns are successively printed to achieve the printing of a band. Then, the paper sheet 15 is travelled in a sub-scanning direction (Y) to perform the printing of a second band next to the first band. This process is iterated so as to form an image constituted by the multitude of bands.
In recent years, a plurality of heads of different ink colors (e. g., cyan, magenta, yellow and black, etc.) are used together so that the different colors of ink are superimposed to form a full-color image. The full-color image requires printed positions of the respective colors (ink ejected positions) to be precise. For this purpose, usually as shown in FIG. 10, a linear scale 301 having slits 304 for every dot position formed therealong and a linear sensor 302 for optically detecting the presence/absence of the slit are used for providing the synchronization for ejecting ink drops, while counting pulse outputs (corresponding to the slits) from the linear sensor 302 for calculating a distance travelled by the head, thereby recognizing the exact positions to be printed on.
Also, the presence/absence of a paper sheet is detected with a paper sensor 303 which is mounted near the heads. As shown in FIG. 11, the paper sensor 303 is scanned on a paper sheet together with a carriage 120. When the paper sensor sequentially detects the left and right edges of the paper sheet, slit-count values are read out which are obtained by counting the output pulses from the linear sensor 302 and which correspond to the respective distances travelled from a reference position, thereby recognizing where in the horizontal direction and what size of paper sheet has been loaded. For such a paper sensor 303, usually a light-reflection type sensor is used which emits light outwardly and detects any reflected light.
In the present specification, the left and right edges of the paper sheet correspond respectively to the left and right sides when viewed from the upstream of the transfer of the paper sheet. Thus, it should be noted that the left and right are opposite to those when viewed from the front of the device.
In forming an image on a paper sheet, the print start and end positions, i.e., margins in the horizontal direction, are determined according to a current position of the paper sensor 303 and the respective heads, based on the position of the paper sheet (count values of the slits of the linear scale 301), taking into consideration a margin from the paper edge Pe and distances of the respective heads from the paper sensor. For example, in FIG. 11, suppose that "A" is a distance between the paper edge position Pe and the paper sensor 303 at the reference position and an amount of margin Z is to be obtained. Then, when the paper sensor 303 reaches a position Pk of [A+Z+X] the printing is started with the K head (black head being the first one in the printing direction). Then, when the paper sensor 303 reaches a position Pc of [A+Z+X+Y1] the printing is started with the C head (cyan head being the second one in the printing direction). Further, when the paper sensor 303 reaches a position Pm of [A+Z+X+Y1+Y2] the printing is started with the M head (magenta head being the third one in the printing direction). Likewise, when the paper sensor 303 reaches a position Py of [A+Z+X+Y1+Y2+Y3] the printing is started with the Y head (yellow head being the fourth one in the printing direction). In this way, adequate margins are ensured and the print start positions of the heads are controlled to be at the same position (Ps).
When performing two-way printing, the similar control also applies to the back path.
Instead of the combination of the linear scale 301 and the linear sensor 302, an alternative means to determine the image start position at the edge of a paper sheet can be realized by counting moving steps of a motor which drives the carriage 20 to move in the X direction.
In the meantime, paper (recording medium) includes normal paper, coated paper, film paper, intermediate paper (tracing paper), etc. and the light reflectance differs depending upon the characteristics of the paper. Now, assume that as shown in FIG. 12, a fixed threshold level (represented by a dashed line) Th is used for a binary conversion to detect a paper sheet with respect to an output from the light reflective type paper sensor 303. In this case, the output level of the sensor will vary depending upon the magnitude of the reflectance. For example, as compared to the sensor output Son with respect to the normal paper which exhibits a normal level of reflectance, the sensor output Sol for paper of a lower reflectance will decrease in its output level. This results in that the binary signal Bl for the paper of the lower reflectance will rise later than the binary signal Bn of the sensor output for the normal paper. Opposite to this, the binary signal Bh for the paper of a higher reflectance will rise earlier than the binary signal Bh for the paper of the normal reflectance. As a result, as shown in FIG. 13, the print start position in the horizontal direction X (main scanning direction of the head 103) for the lower reflectance paper (FIG. 13(b)) will be ahead of that of the normal paper (FIG. 13(a)), and the print start position in the horizontal direction X for the higher reflectance paper (FIG. 13(c)) will be behind of that of the normal paper. This will cause the margins Z1, Z2 and Z3 from the paper edge Pe to the print start positions to vary depending upon the type of paper. In this way, inaccurately detected position of the paper edge affects the accuracy of the margins.
Even when the same type of paper is used, the sensor output level will vary with an ambient temperature depending upon the temperature characteristics of the receiving element of the sensor. This can make the detected position of the paper edge incorrect, changing the margins.
Further, some types of paper could swell and heave as the print density increases, causing the heads to rasp the surface of the paper sheet. To overcome this, there is a device of a type in which a user can arbitrarily adjust the spacing between the heads and the paper sheet. In this type of the device, the paper sensor 303, which is mounted near the heads, will change in its height together with the heads. Thus, the change of the spacing between the paper sheet and the paper sensor 303 will cause the amount of incident light of the sensor to vary, changing its output level so that the detected position of the paper edge becomes inaccurate to change the margins.
It is, therefore, an object of the present invention to provide an image forming device capable of accurately detecting a paper edge position even when states of events change, which causes an output deviation of a recording medium detection means such as a paper sensor.
It is another object to provide an image forming device capable of accurately defining a horizontal margin by accurately detecting a paper edge position.